The disclosure of Japanese Patent Application Nos. 2000-79388 filed on Mar. 22, 2000 and 2000-383485 filed on Dec. 18, 2000 including the specification, drawings and abstract are incorporated herein by reference in their entirety.
1. Field of the Invention
The invention relates to a hydrogen generating system and a method which reforms liquid raw materials to generate hydrogen rich gas, and to a vaporizer utilized in the system.
2. Description of the Related Art
Hydrogen to be supplied to systems, such as fuel cells, in which hydrogen is consumed is generated by reforming liquid raw materials, for example. As the liquid raw materials, liquefied natural gas, gasoline, other hydrocarbons, alcohols, ethers and aldehydes and the like are used in general. In a hydrogen generating system, these raw materials and water are vaporized by a vaporizer and made to undergo a reforming reaction in the presence of a catalyst such as platinum to thereby generate hydrogen rich reformed gas. In order to run the reaction stably, the reforming reaction is maintained in a predetermined temperature and pressure condition by feedback control or the like. This reformed gas is treated to decrease the concentration of components, such as carbon monoxide, and then supplied to a hydrogen consumption system, for example, a fuel cell.
In the hydrogen generating system, the quantity of hydrogen to be generated must follow the quantity of hydrogen to be consumed in a hydrogen consumption system. It is known that the rate-determining step as to the quantity of hydrogen to be generated is vaporization in a vaporizer. Therefore, an improvement in the speed of response of vaporization is required to improve the speed of response of the generation of hydrogen.
The following technologies are proposed with the intention of improving the speed of response of vaporization. For instance, a structure in which the vapor generated during a low load operation is accumulated in an accumulator and the quantity of vapor to be generated is compensated for by the accumulated vapor during a high load operation is disclosed in Japanese Patent Laid-Open Publication No. HEI 8-121705. Also, a structure in which vapor is always generated in a quantity much greater than the quantities required at each point in time is disclosed in Japanese Patent Laid-Open Publication No. HEI 2000-119001.
However, the above systems give rise to the following problems concerning an improvement in the speed of response of the quantity of hydrogen to be generated. In the structure described in Japanese Patent Laid-Open Publication No. HEI 8-121705, the accumulator constitutes an obstacle to the miniaturization of equipment. In recent years, a method in which a hydrogen generating system and a fuel cell are mounted on a mobile body such as a vehicle has been investigated. In such a case, because very severe restrictions are imposed on the mount space, there is a strong demand for miniaturization. Also, in the structure described in Japanese Patent Laid-Open Publication No. HEI 8-121705, a reduction in the temperature of the accumulated vapor must be suppressed to allow the reforming reaction to proceed efficiently, thus making the system more complex. Also, the structure described in Japanese Patent Laid-Open Publication No. HEI 2000-119001 has the problem of low energy efficiency because of the generation of excess vapor.
The systems disclosed in above publications also give rise to the following problems caused by pressure control in the reforming section, particularly at start-up. First, there a pressure control valve tends to be excessively restricted to maintain target pressure when there is an insufficient amount of reformed gas. Therefore, when generation of reformed gas is started, there is a possibility that delay of pressure control may cause the pressure in the reforming section to exceed the target value. Second, a rather high pressure is maintained at a relatively low temperature condition causing vaporized raw materials to be condensed and there is a possibility of the generated liquid adhering to the catalyst thereby decreasing the activity of the catalyst.
It is an object of the invention, first, to improve the speed of response of the quantity of hydrogen to be generated by improving the speed of response of the vaporization of raw materials and the like in a hydrogen generating system. Together with this improvement in response characteristics, the object of the invention is also to attain miniaturization of the equipment, to improve energy efficiency and to maintain the temperature of the vapor. It is an object of the invention, second, to provide a technology for avoiding the problems of vapor, which are caused by pressure control at start-up in the hydrogen generating system.
In the invention, at least part of the foregoing problems has been solved by the improvement in pressure control in a hydrogen generating system. In other words, conventionally, pressure in a hydrogen generating system is controlled such that it is kept in such a constant condition suitable for reformation and the like. The invention, however, adopts a structure in which pressure in the system is changed according to the operational conditions of the system at each point in time or the operational conditions required as shown below.
A hydrogen generating system which is a first embodiment of the invention comprises a vaporizing section, a reforming section and a pressure regulator, and further comprises a controller for controlling the pressure regulator on the basis of a quantitative requirement for hydrogen to be generated.
The vaporizing section is a unit for vaporizing liquid raw materials and is constructed of, for example, a vaporizer. The reforming section is a unit which reforms the vapor supplied from the vaporizing section. The reforming section includes a variety of units used to conduct a chemical reaction for generating hydrogen rich gas from raw materials. These units include a reforming unit which generates hydrogen and carbon monoxide (CO) by the vapor-reforming or partial oxidation of raw materials, a shift reaction unit which generates hydrogen and carbon dioxide by a shift reaction from carbon monoxide and water and a CO oxidation unit which selectively oxidizes carbon monoxide. The reforming section is provided with at least one of these units.
As described above, the speed of response of the generation of hydrogen in the hydrogen generating system is determined by the rate of vaporization. As commonly known, the rate of vaporization is affected by the pressure in the vaporizing section. According to the invention, not only the rate of vaporization but also the speed of response of the generation of hydrogen can be improved by controlling the pressure in the vaporizing section according to the quantitative requirement for hydrogen to be generated. Also, no large-scale equipment such as an accumulator is required and an improvement in the speed of a response can be achieved. It is also unnecessary to accumulate vaporized gas and therefore there is no problem due to the lowered vapor temperature. Moreover, because it is also unnecessary to generate excess vapor, energy efficiency can also be improved.
In the first embodiment, a pressure regulator may be provided in the vaporizing section. It is preferable to adopt a structure in which the vaporizing section is provided with a vapor generating section and a vapor heating section where the pressure in the vapor generating section is regulated by the pressure regulator. The vapor generating section is supplied with liquid raw materials while the pressure therein is regulated with a pressure regulator, forming a vapor-liquid mixed section of the raw materials. The vapor heating section is connected to a vapor phase portion of the vapor generating section and heats the raw materials of the vapor phase portion. This structure makes it possible to obtain a vapor having a desired temperature relatively easily.
Regulation of the pressure in the vapor generating section can be achieved, for instance, by disposing the pressure regulator in the connecting portion between the vapor generating section and the vapor heating section.
Also, the pressure regulator may be disposed downstream of the aforementioned vaporizing section. The pressure regulator maybe disposed, for instance, between the vaporizing section and the reforming section, inside of the reforming portion, and downstream of the reforming section, namely anywhere between the reforming section and a hydrogen consumption system. Since the vaporizing section is communicated with the reforming section, the pressure in the vaporizing section can be controlled even in these positions. These positions are also advantageous in that the pressure in the reforming section can be regulated together with regulation of the pressure in the vaporizing section.
In the case of disposing the pressure regulator inside of the reforming section, when the reforming section is provided with a first unit and a second unit disposed downstream of the first unit, the pressure regulator may be disposed between these units. Here, the first unit is a unit which generates a reformed gas containing hydrogen and carbon monoxide by a reforming reaction of the raw materials. The reforming unit described above, for example, corresponds to this unit. The second unit is a unit which decreases the quantity of carbon monoxide to be produced. The shift reaction unit or CO oxidation unit described above, for example, corresponds to this unit. In this structure, the pressure in the first unit can be controlled to a pressure higher than that of the second unit. This structure also is also advantageous in that the temperature of gas can be reduced by making use of adiabatic expansion when the reformed gas generated in the first unit is transferred to the second unit.
Control in response to the quantitative requirement for hydrogen to be generated can be achieved with various embodiments. For instance, an embodiment in which the pressure in the vaporizing section is decreased according to an increase in the quantitative requirement or a variation in the quantitative requirement may be adopted. Generally, vaporization can be promoted by decreasing pressure. When the intent is to promote vaporization constantly, the pressure may be decreased according to the quantitative requirement. When improving a transient response after the quantitative requirement is increased, the pressure may be decreased according to the variation in the quantitative requirement. It is possible to control the pressure in consideration of both the quantitative requirement and the variation in the quantitative requirement.
Pressure may be controlled by increasing the pressure in the vaporizing section according to the reduction in the quantitative requirement or the rate variation in the quantitative requirement. The reduction in the rate of variation includes both the case where when the rate of variation is positive its absolute value is decreased and the case where when the rate of variation is negative its absolute value is increased. This ensures that the generation of excess vapor can be suppressed rapidly, and thereby improving energy efficiency.
When the pressure in the vaporizing section is raised, heat can be accumulated as internal energy of the container and liquid raw materials because the boiling point of the raw material is raised. Then, by decreasing the pressure in the vaporizing section, vapor of the liquid raw materials can be generated using this internal energy. Therefore, when an increase in the generation of vapor is required, the quantity of vapor can be increased instantly by decreasing the pressure in the vaporizing section, whereas when a decrease in the generation of vapor is required, the quantity of vapor can be decreased instantly by increasing the pressure in the vaporizing section. Specifically, a load change can be dealt with rapidly.
In the first embodiment, hydrogen rich gas is generated as fuel to be supplied to a driving source of a mobile body and the first embodiment is therefore highly useful for an onboard system mounted on the mobile body. This is because the mobile body is strictly limited in its mount space and varies in the quantitative requirement relatively greatly. When the first embodiment is structured as an onboard system, the quantitative requirement may be determined based on, for example, the driving force requirement of the mobile body. It is to be noted that the mobile body includes vehicles, marine vessels, airplanes and flying bodies.
A hydrogen generating system which is a second embodiment of the invention comprises, as a second structure, a reforming section and a pressure regulator which regulates the pressure in the reforming section and further comprises a controller which controls the pressure by properly using at least two different modes. The two modes include a first control mode in which the reforming section is made to have a predetermined target pressure and a second control mode which is executed at the start-up of the hydrogen generating system unlike the first control mode. It is to be noted that the second embodiment may be applied not only to systems using liquid raw materials but also to other systems.
It is preferable that the pressure in the reforming section be maintained in a predetermined condition suitable for the promotion of a reaction when the hydrogen generating system is operated. At the start-up of the system, however, reformed gas is insufficiently generated and the temperature of the reforming section is low. Therefore in this condition, the reaction proceeds with difficulty. Conventionally, pressure control at start-up was not focused on at all. However, in such a condition, it is not always preferable to maintain the same pressure condition as that during regular operation. In the second embodiment, the control mode is switched at start-up and at regular operation such that pressure control suitable respectively to both operations can be attained.
In the second embodiment, for instance, the first control mode is designed to be feedback control in consideration of the time integral of the deviation between the aforementioned target pressure and actual pressure. And as the second control mode, a mode in which the influence of the time integral on the controlled variable is suppressed may be used. For example, the restraint of the influence of the time integral can be attained by decreasing the control gain for the time integral more than in the first control mode. Feedback control excluding the time integral term may also be provided.
Generally, the time integral term produces the effect of maintaining the past condition and smoothing variations in the controlled variable in the feedback control. Because the quantity of reformed gas to be generated is small at start-up of the system, the pressure regulator is controlled in the direction in which the pressure in the reforming section rises. If this condition is sustained for a long period of time, the response of the pressure regulator is delayed, affording the possibility of a rapid increase in the pressure in the reforming section in the case where the influence of the time integral term is large when generation of reformed gas has started. This phenomenon can easily be avoided by suppressing the influence of the time integral term.
For instance, the first control mode may be closed-loop control and the second control mode may be open-loop control. This restrains the pressure regulator from being operated excessively in the direction in which the pressure in the reforming section rises, and therefore a rapid increase in pressure after generation of reformed gas has started can be avoided. As the simplest open-loop control, the pressure regulator is designed to be maintained in a constant condition, for example, an open condition irrespective of the pressure in the reforming section.
The second embodiment of the invention may be further provided with a transfer control mode which suppresses the variation of pressure in the reforming section within a predetermined range when the system is transferred from the second control mode to the first control mode. In the case where the pressure in the reforming section is relatively low when the system is transferred to the first control mode, there is a possibility that the manipulated variable of the pressure regulator will be overshot and cause a rapid rise in pressure transitionally. However, this can be avoided with the provision of the transfer control mode. For the transfer control mode, a method in which the target value of pressure is decreased more than it was originally, a method in which a so-called xe2x80x9csmoothing treatmentxe2x80x9d is provided for the control variable obtained in the first control mode, and a method in which the control variable is set in an open-loop and the like may be applied.
In addition to the above control, feedback control is applied together with the first control mode and the second control mode where the target value of pressure in the second control mode may be lower than in the first control mode. In the second control mode, an upper limit may be given to the manipulated variable in the pressure regulator.
In the second structure of the invention, for example, switching from the first control mode to the second control mode and vice versa may be conducted based on the quantity of state of gas in the aforementioned reforming section. As the quantity of state, the temperature and pressure in the reforming section, the components of the gas and the flow rate of the gas flowing out from the reforming section may be used either singly or in combination. For instance, when the temperature, pressure and flow rate are below the predetermined values respectively, the operation condition is determined as the start-up condition and therefore the second mode is applied. When these parameters are above the predetermined values respectively, the system can be switched to the first control mode. As the components of the gas, components such as hydrogen or carbon monoxide which vary in quantity according to the progress of the reaction in the reforming section are used and the system mode may be switched based on whether the concentration of each of these components is more than the predetermined value or not. These predetermined values as the standard for judging whether the system mode is switched or not can be set based on experiments or the like in advance according to the system structure. These quantities of state may be detected directly in the reforming section or indirectly at a portion, for example, downstream of the reforming section.
In the invention, a pressure regulating valve, a flow metering valve or the like may be used as the pressure regulator. It is preferable to use an electromagnetically controllable valve.
In the hydrogen generating system of the invention, for example, hydrocarbon type compounds may be used as the raw materials. Such compounds include liquefied natural gas, gasoline, other hydrocarbons, alcohols, ethers and aldehydes.
In the invention, the aforementioned various additional elements can be applied by appropriately combining them. Also, structural elements given in the first and second structures may be combined to constitute a single hydrogen generating system.
The invention can be structured with various embodiments in addition to the structure as the aforementioned hydrogen generating system. For example, the first embodiment of the invention may be structured as a vapor generator applied to the hydrogen generating system. The invention may also be structured as a control method which attains the control exemplified in the first and second structures for the hydrogen generating system. Other than the above, the invention may be structured as a fuel cell system in which the hydrogen generating system of the invention is combined with a fuel cell that generates electricity using hydrogen generated in the hydrogen generating system. The invention may also be structured as a mobile body (e.g., a vehicle) mounted with such a fuel cell system.